#ifndef LEVELDB_THREAD_POOL_HH
#define LEVELDB_THREAD_POOL_HH

#include <vector>
#include <queue>
#include <memory>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <future>
#include <functional>
#include <stdexcept>

namespace leveldb {

class ThreadPool {
 public:
  explicit ThreadPool(size_t);
  template<class F, class... Args>
  auto enqueue(F&& f, Args&&... args)
  -> std::future<typename std::result_of<F(Args...)>::type>;
  ~ThreadPool();
 private:
  // need to keep track of threads so we can join them
  std::vector< std::thread > workers;
  // the task queue
  std::queue< std::function<void()> > tasks;

  // synchronization
  std::mutex queue_mutex;
  std::condition_variable condition;
  bool stop;
};

// the constructor just launches some amount of workers
inline ThreadPool::ThreadPool(size_t threads)
    :   stop(false)
{
  for(size_t i = 0;i<threads;++i)
    workers.emplace_back(
        [this]
        {
          for (;;)
          {
            std::function<void()> task;
            {
              std::unique_lock<std::mutex> lock(this->queue_mutex);
              this->condition.wait(lock,
                                   [this]{ return this->stop || !this->tasks.empty(); });
              if(this->stop && this->tasks.empty())
                return;
              task = std::move(this->tasks.front());
              this->tasks.pop();
            }
            task();
          }
        }
    );
}

// add new work item to the pool
template<class F, class... Args>
auto ThreadPool::enqueue(F&& f, Args&&... args)
-> std::future<typename std::result_of<F(Args...)>::type>
{
  using return_type = typename std::result_of<F(Args...)>::type;

  auto task = std::make_shared< std::packaged_task<return_type()> >(
      std::bind(std::forward<F>(f), std::forward<Args>(args)...)
  );

  std::future<return_type> res = task->get_future();
  {
    std::unique_lock<std::mutex> lock(queue_mutex);

    // don't allow enqueueing after stopping the pool
    if(stop)
      throw std::runtime_error("enqueue on stopped ThreadPool");

    tasks.emplace([task](){ (*task)(); });
  }
  condition.notify_one();
  return res;
}

// the destructor joins all threads
inline ThreadPool::~ThreadPool()
{
  {
    std::unique_lock<std::mutex> lock(queue_mutex);
    stop = true;
  }
  condition.notify_all();
  for(std::thread &worker: workers)
    worker.join();
}
}


#endif //LEVELDB_THREAD_POOL_HH
